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Our Fourth Lab: Using Pulse Width Modulation (PWM) to Dim an LED

Parts for this Lab are in Bag 2

  1. We only need a single LED for this lab. As a reminder for the setup:

    Insert the Anode(+) lead into j20

    Insert the Cathode(-) lead into the Ground(-) rail

    Insert the resistor leads into j3 and into the Ground(-) rail

  2. When you are finished, your breadboard should look similar to the image below (Note: your LED may be a different color)

Lab 4

What is Pulse Width Modulation (PWM)?

The object of this lab is to have the external LED start out on. Then, slowly dim the LED. Once the LED is off, slowly bring it back to full brightness. Continue until you stop the program.

Dimming Bulbs

  • We used a switch in the previous labs to turn an LED on and off, but what if we wanted to dim the LED? Since we can’t control the voltage through the GP pins of our device, we have to use PWM. PWM means that we will quickly vary the amount of time (duty cycle) that the voltage is applied to the LED.

PWM

PWM - How Does It Work?

  • PWM is controlled by two factors:
    • How fast does the power turn on and off (frequency)
    • How long is the power on, relative to the power being off (duty cycle)
  • We can declare that a GP pin is a PWM pin as follows: 
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    from machine import Pin, PWM
pwmPin = PWM(Pin(16))
  • Now set the frequency and the duty cycle: 
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    # Apply and remove power 1000 times/second (too fast to see)
pwmPin.freq(1000)

# Apply voltage 50% of the time
# Range is 0 - 65535
pwmPin.duty_u16(32768)

Challenge

Now that we understand PWM, create a program that starts with the LED off, slowly brings the LED to full brightness, and then slowly back to off.

Once that works, what happens if we repeat the exercise, starting with a frequency of 1000, and reduce the frequency to 10? Use a list with values: 1000, 500, 100, 50, 40, 30, 20, & 10.

  • Don't forget to comment your code!